Symptomatic lower extremity deep venous thrombosis ...download.xuebalib.com/1l926CU0m2Oi.pdflimbs...
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Steven C. Rose, MD #{149}William J. Zwiebel, MD #{149}Brent D. Nelson, MD #{149}Derek L. Priest, RVT .
Rhonda A. Knighton, RVT #{149}Joyce W. Brown, LPN-VT #{149}Peter F. Lawrence, MDBarry M. Stults, MD #{149}James C. Reading, PhD #{149}Franklin J. Miller, MD
Symptomatic Lower Extremity Deep VenousThrombosis: Accuracy, Limitations, and Roleof Color Duplex Flow Imaging in Diagnosis’
From the Departments of Radiology (S.C.R., W.J.Z., B.D.N., D.L.P., R.K., J.W.B., F.J.M.), Surgery
(P.F.L.), Internal Medicine (B.M.S.), and Family and Preventive Medicine (J.C.R.), University ofUtah Medical Center, 50 N Medical Dr. Salt Lake City, UT 84132; and the Veterans AdministrationMedical Center, Salt Lake City. From the 1989 RSNA annual meeting. Received November 28,1989; revision requested January 19, 1990; revision received February 2; accepted February 20. Ad-dress reprint requests to S.C.R.
C. RSNA 1990
639
Cardiovascular Radiology
Color duplex flow imaging (CDFI)permits pain- and risk-free directimaging of the deep venous systemof the lower extremities. To pro-spectively ascertain the accuracyand limitations of this technique,CDFI was performed in 75 lowerlimbs of 69 consecutive patients re-ferred for venographic evaluation ofclinically suspected lower extremitydeep venous thrombosis (DVT). TheCDFI study was obtained within 24hours of the contrast venogram.Both studies were interpreted with-out knowledge of the patient’s clini-cal findings or the results of theother test. Contrast venography wasregarded as the standard for diagno-sis of DVT. Accuracy was 99% fordetection of DVT above the kneeand 81% below the knee. Sono-graphic evaluation of the calf veinswas technically adequate in 60% oflimbs; accuracy was 98% in thisgroup. In the 40% of limbs withtechnically limited CDFI studies ofthe calf, accuracy decreased to 57%.Although small nonocclusivethrombi occurred infrequently inthis series of symptomatic patients,CDFI missed three of four suchthrombi. It is concluded that CDFI,when not technically compromised,is sufficiently accurate to defini-tively diagnose symptomatic lowerextremity DVT.
Index terms: Extremities, thrombosis, 93.751 #{149}
Extremities, US studies, 44.12984, 45.12984
Thrombosis, US studies, 93.12984 #{149}Thrombo-
sis, venous, 93.751 #{149}Ultrasound (US), Doppler
studies #{149}Veins, stenosis or obstruction, 93.751 #{149}
Veins, US studies, 93.12984
Radiology 1990; 175:639-644
D EEP venous thrombosis (DVT) of
the lower extremities is a com-
mon and serious disorder that re-
quires diagnosis and treatment be-
fore life-threatening complications
(eg, pulmonary embolization) occur.
Unfortunately, the primary methods
for diagnosis of DVT have been sub-
stantialiy underutilized because of
associated pain and complications
(contrast venography), suboptimal
diagnostic accuracy (most noninva-
sive techniques), or logistic encum-
brances (serial impedance plethys-
mography or radiofibrmnogen uptake
tests) (1). Postvenography phlebitis is
reported to occur in 8%-9% of pa-
tients, approximately one-third of
the cases involving the deep venous
system (2,3).
Color duplex flow imaging (CDFI)
displays blood flow within vascular
structures in colors. The color and
hue depend on the velocity and di-
rection of blood flow relative to the
transducer. Veins are readily distin-
guished from arteries by flow direc-
tion and Doppler flow characteris-
tics. Thrombus is identified as a flow
void within the imaged lumen (Fig
1). Because CDFI can directly demon-
strate most of the deep venous sys-
tern in the lower extremities, we hy-
pothesized that this technique could
enable detection of symptomatic
DVT with an accuracy close to that of
contrast venography and substantial-
ly better than the reported accuracy
of other noninvasive techniques. Us-ing contrast venography as the refer-
ence test, we evaluated the accuracy
and technical limitations of CDFI in
69 consecutive patients with clinical-
ly suspected lower extremity DVT.
MATERIALS AND METHODS
All patients with symptoms or signs
(pain, tenderness, or swelling) suggesting
lower extremity DVT who underwentsuccessful contrast venography, regard-
less of result, were offered CDFI at no ad-
ditional cost, provided the latter study
could be performed within 24 hours of
venography. Nineteen patients were ex-
cluded from the study because elective
CDFI was not performed within 24 hours.All patients offered free CDFI consented
to be evaluated. This study included 69
consecutive patients who underwent
both venography and CDFI. Bilateral
studies were performed in six patients
with bilateral symptoms; therefore, a total
of 75 lower extremities were evaluated. In
no case was the same extremity studied
more than once. The pelvic portion ofone extremity and the calf portion of a
second limb were excluded from the
study because those segments of the yen-
ograms were technically inadequate.
The venographic technique was a mod-
ification of that of Rabinov and Paulin
(4). Specifically, the patient was tilted onthe fluoroscopic table to at least 30#{176}re-
verse Trendelenburg (head elevated). In
all patients, both tight-tourniquet (ankle
and knee) and tourniquet-free views of
the calf were obtained. Venous opacifica-
tion was fluoroscopically monitored and
documented with overhead radiographs.On average, 120-150 mL of contrast mate-
rial (Conray 43%; Mallinckrodt, St. Louis)
was used in each lower extremity. Thestudy was concluded when the entire
deep venous system was demonstratedfrom the ankle to the iliocaval junction.
After all imaging was completed, the ye-
nous system was irrigated with 250 mL of
heparinized saline (4 U/mL).The sonographic technique used was a
modification of that published by Talbot
(5). A high-resolution CDFI ultrasound
(US) unit (Quantum Angiodynograph I;
Quantum Medical Systems, Issaquah,Wash) was used with a 3.0-, 5.0-, or 7.5-
MHz transducer coupled to a wedge-shaped water path. Transducer selection
depended on patient habitus and the
Abbreviations: CDFI color duplex flowimaging. CI = 95% confidence interval; DVT
deep venous thrombosis.
2. 3.
Figures 2, 3. (2) Contrast venography demonstrates a nonocclusive 2-cm-long thrombus(arrowheads) within a branch of the profunda femoris vein in a paraplegic patient with legswelling and fever. CDFI, which missed this thrombus, was generally not used in our series
to evaluate the deep portions of the profunda femoris vein. (3) Venogram shows nonocclu-
sive soleal sinus thrombus (arrowheads) missed on a technically adequate CDFI study. Al-
though the soleal sinuses are not routinely imaged on venous sonograms, new-onset focal
calf pain and swelling 4 days after knee arthroplasty should have directed attention to thispossibility.
640 #{149}Radiology
Figure 1. CDFI display of blood flow. Col-or-encoded flow allows unequivocal differ-entiation of vessels (colored) from surround-
ing tissues (gray), arterial flow (red) from
venous flow (blue), and patent venous lu-mens (blue) from thrombosed venous lu-
mens (black; ie, anechoic thrombus).
thickness of overlying soft tissues. Instru-ment controls (software mediated) wereset to detect low (minimum, 0.3 cm/sec)or medium (minimum, 0.7 cm/sec) flow
rates, adjusted to the venous flow velocityof each patient. The patient lay supine onan examination table tilted to 20#{176}-30#{176}re-verse Trendelenburg (head elevated). Theinvolved lower extremity was rotated ex-ternally 30#{176}-45#{176}with 20#{176}-40#{176}of flexionat the hip and knee. Typical examinationtime was 20-30 minutes per extremity.
Direct CDFI of the iliac veins was per-formed with the 3.0- or 5.0-MHz trans-ducer. Because of the depth of the iliacveins and their obscuration by overlyingbowel gas, only 3-5 cm of external iliacvein could be imaged in most patients. Il-iac vein patency was further assessed bymeans of CDFI study of flow signals inthe common femoral vein. Normal flowin the common femoral vein characteristi-
cally produces spontaneous centripetalDoppler signals, with respiration-in-duced modulation of flow velocity, cessa-tion of flow during a vigorous Valsalvamaneuver, and augmentation of flowwith compression of the calf.
Below the inguinal ligament, the 5.0- or7.5-MHz transducer was used. Longitudi-nal image planes over the anteromedial
thigh were used to evaluate common fem-
oral, superficial femoral, deep femoral,and greater saphenous vein anatomy, pa-tency, and blood flow characteristics. Thetransverse transducer orientation was usedto confirm anatomic relationships and to
assess vein compressibility (coaptation of
vein walls with minimal pressure).
The distal superficial femoral and pop-liteal veins were evaluated with the
transducer placed posteriorly in the pop-liteal fossa. The posterior tibial and pero-
neal veins were examined with the trans-
ducer positioned posteromedially along
the calf. Longitudinal image planes were
used in the popliteal region and calf toevaluate venous anatomy, patency, andaugmentation of venous flow with foot or
distal calf compression. Transverse trans-
ducer orientation in the popliteal area
was used to confirm anatomic relation-ships and to assess vein compressibility.
The anterior tibial veins were examined
only if anterolateral calf symptoms were
present. The anterior tibial veins were ex-amined in longitudinal sections, with the
transducer positioned anterolateral to the
tibia. In addition to examination of the
deep venous system, regions of pain, ten-derness, or local swelling were evaluatedfor evidence of superficial venous throm-
bosis or nonvenous disease.
CDFI of the veins was performed byone of three experienced vascular tech-
nologists. The studies were recorded on
videotape for blinded interpretation at alater time.
Contrast venograms were independent-ly double-read by two experienced angio-graphers (5CR., F.J.M.) who were blind-ed to both the clinical symptoms and in-
terpretation of the CDFI studies. Discor-
dant interpretation occurred infrequently
and was resolved by consensus. Similarly,
the CDFI studies were interpreted by one
of three experienced reviewers (W.J.Z.,P.F.L., F.J.M.) who were blinded to bothclinical information and venographic in-
terpretation.
Thrombosis was diagnosed venogra-
phically if there was a consistently
present intraluminal filling defect or seg-
mental venous occlusion that was not at-
tributable to venographic technique (eg,
tourniquet or needle placement). CDFI
diagnosis of thrombosis was based pri-
manly on the presence of a focal void
within the color-encoded blood flow im-
age or the absence of visible flow withina segment of a vessel. Usually the flowvoids were accompanied by intraluminalechogenicity, vein distention, and ab-
sence of normal vein compressibility. For
the purposes of this study, no attempt
was made to estimate thrombus age.Patient charts were retrospectively re-
viewed to confirm presenting symptomsand to determine which patients had a
history of DVT. Forty-seven of the pa-
tients (68%) were men, and 22 (32%) were
women. Mean patient age was 54 years(range, 21-92 years). Presenting symp-
toms included swelling (69 limbs), pain
(55 limbs), tenderness (28 limbs), erythe-
ma (eight limbs), and palpable cord (two
June 1990
a. b. c.
Volume 175 #{149}Number 3 Radiology #{149}641
Figure 4. Variable image adequacy of CDFI studies of the calf. (a) Technically adequate image of the posterior tibial and peroneal pairedbranch veins. (b) Increased sound attenuation caused by excessive accumulation of soft-tissue edema (hypoechoic subcutaneous striations)
impairs imaging of deep structures. (c) Multiple small collateral veins (red and blue dots) in a patient with previous DVT cause confusion
when an attempt is made to distinguish and follow the main conduit veins of the calf. In addition, the multiple acoustic interfaces contribute
to sound attenuation.
limbs). The mean duration of symptoms
was 14 days (range, i day to 6 months).Thirty patients (43%) were inpatients, and
39 (57%) were initially evaluated as out-
patients. DVT was found with venogra-
phy in 32 of the 75 (43%) lower extrem-
ities examined. For purposes of this
study, each lower extremity examination
was tabulated as if it were an indepen-dent event.
Ninety-five percent confidence inter-
vals (CIs) were calculated with the tech-nique described by Fleiss (6). Standard
two-by-two x2 tests were performed to
compare accuracies.
Overall
RESULTS
Correct identification of DVT with
CDFI (true-positive cases)-regard-
less of thrombus size, location, or ex-
amination quality-occurred in 27
limbs. Thirty-six limbs were correctly
identified as not having DVT (true-
negative cases). Five CDFI studies
were false-positive relative to venog-
raphy, and seven were false-nega-
tive. Overall sensitivity was 79%,
specificity 88%, positive predictive
value 84%, negative predictive value
84%, and accuracy 84% (CI 73.3%-
91.1%).
Anatomic Location
CDFI results for detection of DVT
above the knee (both iliac and femo-
ropopliteal venous segments) were as
follows: 25 true-positive, 49 true-neg-
ative, no false-positive, and one false-
negative result (sensitivity, 96%;
specificity, 100%; positive predictive
value, 100%; negative predictive val-
ue, 98%; and accuracy, 99% [CI
91 .8%-99.9%}).
For detection of DVT proximal to
the inguinal ligament (common and
external iliac veins), CDFI results
were true-positive in three limbs and
true-negative in 62 limbs, with no
false-positive or false-negative re-
sults. All three cases of identified iii-
ac vein DVT also had noncontiguous
DVT of the femoropopliteal segment.
In nine limbs, common femoral vein
thrombosis precluded assessment of
iliac vein patency with Doppler sig-
nal. The pelvic portion of one pa-
tient’s study was excluded because
the venogram was nondiagnostic
(underexposed film). Isolated iliac
vein thrombosis did not occur in this
series of patients. For the iliac veins,sensitivity, specificity, positive and
negative predictive values, and accu-
racy were 100%.
For detection of DVT between the
ingumnal ligament and knee (corn-
rnon femoral, superficial femoral,
deep femoral, and popliteal veins),
CDFI results were true-positive in 23
limbs, true-negative in 50, false-posi-
tive in none, and false-negative in
two. In one of the false-negative
cases the thrornbus was of small cali-
ber and less than 3 cm in length (Fig
2). The CDFI study in the other false-
negative case was of limited quality
because of the presence of soft-tissue
edema and extensive collateral vein
formation that obscured a segment of
partially recanalized old thrombus in
the distal superficial femoral vein.
For the region between the inguinal
ligament and the knee, sensitivitywas 92%, specificity 100%, positive
predictive value 100%, negative pre-
dictive value 96%, and accuracy 97%.
For detection of thrombus in the
infrapopliteal deep veins (tibioper-
oneal trunk and trunks and branches
of the posterior tibial and peroneal
veins), CDFI results were true-posi-
tive in 22 limbs, true-negative in 38,
false-positive in six, and false-nega-
tive in eight. The calf portion of one
patient’s study was excluded because
numerous overlapped veins rendered
the venogram nondiagnostic for calfDVT. In this region, sensitivity was
73%, specificity 86%, positive predic-tive value 79%, negative predictive
value 83%, and accuracy 81% (CI
70.0%-88.9%).
Eleven patients had venographic
DVT of the anterior tibial veins. In
no case, however, was DVT isolated
to the anterior tibial veins; each of
these patients had concomitant femo-
ropopliteal DVT (extensive in 10 pa-
tients). Supplementary CDFI studies
of the anterior tibial veins were per-
formed in only two patients with an-
terolateral calf symptoms. One study
642 #{149}Radiology June 1990
was technically adequate and had
true-positive results. The other was
technically limited and had false-
positive results.
Calf Studies
The calf was the most difficult re-
gion to visualize with CDFI. In 45 ex-
tremities (60%), CDFI of the calf was
technically adequate (ie, the tibioper-
oneal trunk and all portions of the
posterior tibial and peroneal veins
were imaged, or definite thrombus
was identified). In the group with
technically adequate examinations,
the CDFI study was true-positive in
19 calves, true-negative in 24, false-
positive in none, and false-negative
in one. One calf was excluded be-
cause of a nondiagnostic venogram.
In the false-negative case the nonoc-
clusive thrombus was within a soleal
sinus and was small (less than 3 cm
long) (Fig 3). For this subgroup of ad-
equate calf studies, sensitivity was95%, specificity 100%, positive pre-
dictive value 100%, negative predic-
tive value 96%, and accuracy 98%
(CI = 87.0%-99.9%).
DVT was absent above the knee in
50 limbs (those with potential isolat-
ed calf DVT). Twenty-nine of these
50 extremities (58%) had technically
adequate CDFI studies. Four calves
had true-positive results for DVT, 23
true-negative, none false-positive,
and one false-negative. One calf was
excluded because of a nondiagnostic
calf venogram. For technically ade-
quate CDFI studies of the calf in the
setting of possible isolated calf DVT,
sensitivity was 80%, specificity 100%,
positive predictive value 100%, nega-
tive predictive value 96%, and accura-
cy96%.
In 30 extremities (40%), technical
factors limited the ability to visualize
the deep venous system of the calf
(Fig 4). In these cases, the recorded
causes of compromised visibility
were prominent calf swelling (18limbs), obscuration by numerous ad-
jacent collateral veins (seven limbs),
obesity (three limbs), local pain (one
limb), small-caliber nondistended
vessels in a paraplegic patient (one
limb), an open calf wound (one
limb), overlying wound adhesive
strips (one limb), a combative patient
(one limb), and technical oversight
(one limb). In two extremities, the
reason for poor visualization was not
recorded. Eight calves had more than
one cause for a limited study. Results
in calves with technically limited
studies were true-positive in three
cases, true-negative in 14, false-posi-
tive in six, and false-negative in 5ev-
en. Sensitivity was 30%, specificity
70%, positive predictive value 33%,
negative predictive value 67%, and
accuracy 57% (CI 37.7%-74.0%). A
x2 test comparing the accuracy of ad-equate CDFI studies of the calf (98%)
with the accuracy of limited calf
studies (57%) was statistically signifi-
cant (P < .001).
Twenty-one of the 50 extremities
(42%) without above-knee thrombus
(those with potential isolated calf
DVT) had technically limited studies.
In this group, CDFI results were true-
positive in none, true-negative in 12,
false-positive in five, and false-nega-
tive in four. Sensitivity was 0%,
specificity 71%, positive predictive
value 0%, negative predictive value
75%, and accuracy 57% (CI 34.4%-
77.4%).
Small Thrombi
Four extremities had small-caliber
nonocclusive thrombi less than 3 cm
long on venograms. Two were isolat-
ed thrombi: one in a deep femoral
vein and the other in a large soleal si-
nus. CDFI missed both thrombi. Two
additional cases involved small
thrombi in calf veins of extremities
with concurrent occlusive iliofe-
moral DVT. CDFI showed one of
these small thrombi but missed the
other.
History of DVT
Nineteen patients (28%) (19 lower
extremity studies) had a history of at
least one episode of DVT in the ex-
tremity examined. When thrombus
size, location, and imaging quality
were disregarded, CDFI results were
true-positive for DVT in 1 1 cases,
true-negative in six, false-positive in
none, and false-negative in two. Sen-
sitivity was 85%, specificity 100%,
positive predictive value 100%, nega-
tive predictive value 75%, and accura-
cy 89%. Eleven of the 13 patients
with venographically demonstrated
DVT had above-knee thrombus; all
1 1 thrombi were detected with CDFI.
The quality of the venous study of
the calf in patients with a history of
DVT was deemed adequate in eight
patients (42%). CDFI results were
true-positive in five cases and true-
negative in three, and there were no
false-positive or false-negative re-
sults. CDFI of the calf veins was tech-
nically inadequate in 1 1 patients
(58%), mainly because of calf swell-
ing and the presence of numerous
collateral venous channels. Among
these inadequate calf studies, CDFI
results were true-positive in one case,
true-negative in four, false-positive
in one, and false-negative in five.
DISCUSSION
The clinical diagnosis of DVT is
generally inaccurate, with a sensitiv-
ity ranging from 14% to 78% and a
specificity ranging from 4% to 21%,
depending on the clinical sign being
considered (7-1 1). Contrast venogra-
phy has been accepted as the defini-
tive modality for diagnosis of DVT.
Clinical follow-up has confirmed the
validity of withholding anticoagu-
lant therapy in patients with a nor-
mal lower extremity venogram (10).
Unfortunately, venography is associ-
ated with significant patient discom-
fort and a low level of risk for con-
trast material-induced nephropathy
and phlebitis, contrast material reac-
tions, and skin slough due to extrava-
sation of contrast material at the in-
jection site (2,3,12,13). For these
reasons, venography has been
underutilized and many patients
with lower extremity DVT remain
undiagnosed (1).
In an effort to improve patient
safety and tolerance, multiple nonin-
vasive techniques have been devel-
oped for the detection of DVT. Im-
pedance plethysmography measures
venous capacitance and the rate of
venous outflow from the lower ex-
tremities (14,15). In expert hands, im-
pedance plethysmography has a sen-sitivity and specificity ranging from
87% to 100% and 92% to 100%, respec-
tively, for detection of DVT above
the knee (16). However, these results
have not been consistently reproduc-
ible in other centers (17). Further-
more, the sensitivity of impedance
plethysmography for detection of
isolated calf DVT is only 17%-33%
(18). Multiple radionuclide tech-
niques have been developed but
have not been sufficiently accurate to
replace contrast venography (19-21).
Gray-scale US, usually supple-
mented with Doppler flow assess-
ment, has proved to be accurate for
detection of femoropopliteal DVT
(sensitivity, 88%-100%; specificity,
92%-100%) and occlusive thrombosis
of the common and external iliac
veins (1 1,22-32). One significant lim-
itation of standard duplex sonogra-
phy has been the difficulty many in-
vestigators have experienced in iden-
tifying and therefore assessing the
deep veins of the calf (11,22-32). A
second limitation has been the in-
ability to directly image the common
Volume 175 #{149}Number 3 Radiology #{149}643
and external iliac veins because of
their depth from the skin and the
presence of overlying bowel gas
(31,33). In most studies the patency
of the iliac veins has been assessed
indirectly by Doppler analysis of
blood flow in the common femoral
vein. False-positive results may be
caused by venous compression by
pelvic masses (24,27,33-35). Sources
of false-negative results from gray-
scale duplex studies include nonoc-
clusive thrombi, the development of
sizable collateral venous channels,
and the presence of anechoic throm-
bus (which occurs in as many as 56%
of patients with acute thrombus)
mimicking a patent lumen (36). Lack
of venous compressibility when gen-
tie pressure is applied accurately in-
dicates anechoic thrombus in regions
where the deep venous system lies
near the skin (1,22,24-32). Alterna-
tively, anechoic venous thrombus
may be missed in regions where ye-
nous compression is difficult, specifi-
cally the distal superficial femoral
vein as it courses through the adduc-
tor canal and the deep calf veins
(1,22,24-32,35).
Because CDFI vividly displays ye-
nous blood flow, this technique al-
lows direct imaging of the entire ye-
nous system of the lower extremities.
Unlike other noninvasive methods,
CDFI allows calf veins to be readily
examined in most patients (37,38).
The diagnostic accuracy of CDFI for
detection of above-knee DVT in our
series (99%) compares favorably with
results achieved with other noninva-
sive modalities, particularly gray-
scale sonography. Although iliac
vein patency could not be evaluated
in nine patients, this difficulty was of
no consequence because evaluation
was rendered unnecessary by the si-
multaneous presence of sonographi-
cally identified common femoral
vein thrombus. Since neither isolated
nor nonocclusive iliac vein thrombo-
sis occurred in this series of patients,
the ability of CDFI to enable detec-
tion of such infrequent but potential-
ly important thrombus remains un-
tested. The large caliber and relative-
ly superficial course of the femoro-
popliteal veins allowed a technically
adequate study of the thigh in all pa-
tients but one.
The lower diagnostic accuracy (81%
overall) of CDFI for detection of in-
frapopliteal DVT in this series was
due to a relatively high frequency of
technically limited calf studies (40%).
The most common cause of subopti-
mal calf sonograms was poor sound
penetration due to profound calf
swelling, multiple collateral vessels,
or obesity. When these calf veins
were adequately imaged (in 60% of
extremities), diagnostic accuracy was
98%, which is superior to the results
of other noninvasive techniques. The
clinical importance of DVT in the
calf remains controversial. Some evi-
dence has been presented suggesting
that calf vein thrombosis may giverise to clinically significant pulmo-
nary emboli (39-43). In approximate-
ly 20% of cases of infrapopliteal DVT,
the thrombosis has been shown to ex-
tend proximally above the knee (44).
The presence of above-knee DVT
greatly increases the risk for pulmo-
nary embolism and eventual post-
phlebitic syndrome (45-47). There-
fore, noninvasive modalities that do
not demonstrate isolated calf DVT
with high sensitivity, such as imped-
ance plethysmography, must be re-
peated serially over 7-10 days to de-
tect proximal extension of thrombus.
If our results are confirmed by other
investigators, it may be feasible to
omit noninvasive serial follow-up
testing in patients with clinically sus-
pected DVT but with technically ade-
quate negative CDFI studies.
Although Polak et al were able to
image the anterior tibial veins in 65%
of CDFI studies in the calf (38), we do
not routinely image these vessels.
The fact that all patients with veno-
graphic anterior tibial vein thrombo-
sis had associated, readily detectable
femoropopliteal DVT may support
the omission of this portion of the
CDFI study in patients without anter-
olateral calf symptoms.
A potential diagnostic pitfall of
CDFI is its poor sensitivity for detec-
tion of small nonocclusive thrombi.
CDFI demonstrated only one of four
such thrombi in our patients. In our
series of symptomatic patients, isolat-
ed small nonocclusive thrombi oc-
curred infrequently (two of 69 pa-
tients). We are aware, however, that
this pitfall may represent a major
limitation in screening for asympto-
matic DVT in high-risk patients (eg,
those immobilized for a prolonged
period after major hip or knee sur-
gery), a population shown to have a
high prevalence of nonocclusive
thrombosis (48-49).
One unanticipated result of our
study was the high level of accuracy
of CDFI (89%) for detection of throm-
botic occlusion in the subset of pa-
tients with a history of previous DVT
involving the same extremity. This
favorable outcome was probably the
result of a preponderance of above-
knee DVT in this population. The
high frequency of above-knee throm-
bosis in our series offset the fact that
58% of patients with a history of pri-
or DVT had diagnostically limited
calf studies. Cronan and Leen dem-
onstrated that compression (gray
scale) US has limited value in detect-
ing acute DVT in patients with docu-
mented previous DVT: 53% of ex-
tremities with previous acute DVT
had sonographic abnormalities sug-
gestive of acute DVT on routine fol-
low-up sonograms 6-31 months after
the acute episode of thrombosis (50).
It remains to be seen whether CDFI
can enable differentiation of new
DVT from old.
With respect to our results in limbs
with potential isolated DVT in a calf
vein, it is noteworthy that CDFI tech-
nology continues to evolve. Substan-
tial improvements in image quality are
imminent. These improvements will
no doubt increase the proportion of
satisfactory calf vein studies and the
accuracy of the procedure in general.
In conclusion, on the basis of the
results presented here, we believe
that CDFI may be used as the prima-
ry imaging modality in patients with
lower extremity symptoms suggest-
ing DVT. Contrast venography, with
its attendant discomfort and risks,
can be reserved for (a) patients with-
out sonographic evidence of above-
knee DVT but with technically limit-
ed calf studies, and (b) patients with a
history of DVT and sonographic evi-
dence of DVT of indeterminate dura-
tion. U
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